The invention relates to tensioning systems for cables in prestressed concrete.
A conventional tensioning system for cables made up of strands requires the strands to be cut to a size greater than the required length of the cable. This additional length is required to allow a jack to grip the strands at their free ends and thus tension them by stretching them.
The locking of the strands on the jack and on an anchoring head through which the strands pass to the jack is effected by, for example, means of wedge anchorages.
Such a system has the following disadvantages.
The jack in such a system has substantial size and weight and it is necessary, therefore, to have sufficient space around the ends of the cables to maneuver the jack and this space is not always available. It is also necessary to have lifting means for the jack which remain in use throughout the tensioning of the cable. In addition, the outer dimensions of such jacks cause considerable inconvenience and lengthen the time taken to complete tensioning; for example, during the prestressing of continuous beams with joined cables, or at cantilevered points, the time taken in tensioning the cables determines the rate at which the complete job is performed, and therefore affects the cost of the job.
The additional length of the individual strands is a waste of material, which, in percentage terms, is greater in shorter cables than in longer cables.
The determination of the extent of the final lengthening of the cable, formed, for example of 12 to 31 strands of 0.5" size, is uncertain because of the variations of the bedding of the wedges in the anchoring head, each time the cable is blocked. This uncertainty is greater when the cable is tensioned in several stages. In such cases, every time the cable is blocked at an intermediate stage and then released to continue with the tensioning, there is an increased variation in the bedding of the wedges and the value of the force required to release the wedges. Thus, the value of the final lengthening cannot be estimated with accuracy. Further, the use of steel strands with high breaking stresses, and with final stresses in the strands very close to the yield stress of the strands, makes a knowledge of the exact value of the final elongation even more important, particularly if the cables are short and the percentage incidence of error is quite substantial.
When it is required to slacken completely a cable already stretched to the limit, it is necessary to apply excess tension to the strands to release them from the wedges and, in so doing, particularly if the cable is short, the excess stress may cause the stress in the cables to exceed admissible values. In addition, in such an operation, the use of the single jack used for stretching is not advisable, because it is preferable to slacken the individual strands one by one, using a special small-size jack. Thus, such a slackening operation is lengthy and dangerous.
Modern, sophisticated constructional methods, and the use of materials with ever-increasing strength characteristics, require prestressing systems which are precise, simple, safe and fast.
It is an object of the invention, therefore, to provide a system for tensioning the strands or wires of a cable in which the aforementioned disadvantages are mitigated or overcome.